Semiconducting composition and tape produced therefrom



Patented Apr. 30, 1946 UNITED sTATi-:s PATENT orrics SEMICONDUCTING COMPOSITION AND v TAPE PRODUCED THEREFBOM Harry H. Barker, Irwin, Lawrence R. Hill, Wilkinsbur and Leo J. Berberich, Forest Hills, Pa., assignors to Westinghouse Electric Corporation, East Pitt'sburgh,1a., a corporation oi Pennsylvania Application December 3, 1942, No. 407,729 Claims. (Cl. 174-102) This invention relates to semiccnducting compositions for application -to electrical apparatus in order to prevent corona in high-voltage conductors. I'his application is an improvement on the invention disclosed in the copending patent application Serial No. 467,727 of H. H. 'Barker nled December 3, 1942. c

The formation of corona in electrical apparatus limits the permissible voltages since corona tends to destroy organic insulation on high-voltage members exposed to air-or other gaseous medium. Corona is produced in electrical apparatus when the surface of the electrical insulation thereof exposed to air or other gas is at such an electrostatic voltage gradient that the molecules of air or other gas are rendered highly active chemically and electrically. When subjected to these voltage gradients, air. for example. will form ozone and ynitrous oxides, which react with water vapor to form oxidizing acids that are destructive to organic insulation. Since organic insulation constitutes themajor portion of insulation in objects of the invention, reference may be had to the following detailed description anddrawing, in which:

Figure l is a fragmentary view in elevation partly in section of an electrodynamic apparatus;

Fig. 2 is an enlarged fragmentary cross-sectional view of a tape prepared according to the Y invention: and

. Fig. 3 is a fragmentary view in elevation showing the processof applying the semiconducting coatings of the invention to a conductor.

In building high-voltage electrical apparatus, semiccnducting compositions in the form of paint or tape has been applied as a coating to the exterior surface of the electrical conductors thereof in order to grade the electrostatic potential onv the surfaces exposed to air or other gaseous medium. While in theory the application of such l semiccnducting materials appears simple and most present-day electrical apparatus, corona becomes a limiting factor in the permissible voltage employable in the electrical conductors. The use of voltages above the threshold values at which coronais initiated may result in such deeasy to carry out, unexpected dimculties are met with -in practice, The selection of semiconducting compositions having a suitable range of resistivities to adequately reduce the electrostatic potential below the threshold values at which corona is formed is limited due to the fact that low resistivity compounds will not grade the postructive action upon the organic insulation of' Y conductors that in some cases in only a few months the insulation, if entirely organic, may be destroyed or impaired so greatly that it will short-circuit and become ineffective for its injtended purpose. If inorganic material such as mica is employed-in combinationwith organic substances as insulation, the destruction is limited to the organic insulation." However, this necessitates refinishing the apparatus at frequent intervals. c

'I'he object oi' this invention is to provide for a semiccnducting `composition which may `be readily applied with commercial quality control to high-voltage electrical members, and which is stable under the operating conditions to which obvious, and will, in part, appear hereinafter.

tential suillciently while intermediate resistivity compounds will permit relativeLv enormous currents to fiow along-the surface of the-insulated conductors and give rise to excessive heating. As is well known, high temperatures are as destructive to organic insulation as is corona. It has i been discovered that for conductors having voltages of the order of 6,600 or greater, the semiconducting compositions should have a surface Jresistivity of the order of from 1 to 1000 megohms per square of 3 to 4 nuls thick film in order to avoid excessive heating and to prevent corona. Where a tape is employed in combination with the composition, this order of resistance is obtained by applying suilicient composition to 10 mils tinck tape to produce a iinal thickness of about l2 mils. l

From experiencev and research, it has been found that while certain carbonaceous lsubstances, such as wood chars, may be applied in combination with a vehicle such as varnish to insulated conductors to produce semiccnducting coatings having the resistances of the order of 1 t o 1000 megohms per square of 3 to 4 mils thick illm, such coatings do not maintain a constant resistivity. f The coatings ordinarily increase in resistivity with time at such a rate that in a few years the semiccnducting coating is ineilective For a better understanding ci' the nature and so for its intended purpose.

posed of natural anthracite coal.

Application of semiconducting coatings as a paint to insulated conductors in order to eliminate corona is not favored in certain cases, since painted coatings are so susceptible to injury from fiala ing or the like due to the normal expansiomand contraction of the members, accidental abuse, corrosive conditions, and similar occurrences. It has also been proposed heretofore to employ a tape coated with semiconducting material as the means of applying semiconducting composition to the surfaces of insulated conductors insteadof painted coatings. In practice, it has been found that tape prepared by incorporating a conventional carbonaceous semiconducting material in an organic film-forming resin is not entirely effective for eliminating corona. Between each lap of semiconducting tape prepared in this manner and applied to the insulated conductor, there apparently exists an air gap of such character that corona may be formed therein. This plienomenon is ascribed to the fact that the successive spirals of tape do not contact each other sufficiently to produce an effective electrical conducting relationship.

According to this invention, it has been discovered that members of a group of water soluble organic film-forming materials may be combined with anthracite coal comminuted to substantially colloidal dimensions whereby a semiconducting paint may be prepared. Semiconducting tapes may be prepared by the use of this paint which are not subject to the defects of tapes 'prepared heretofore.

Anthracite coal has a resistivity depending on the source of the coal such that when incorporated in suitable proportions in an organic filmforming material, any surface resistivity within the range of about 1 to 1000 megohms per square of 3 to 4 mils thick fllm may be obtained. The resistivity may be closely controlled by blending anthracite coal from two or more independent sources since the anthracite coal from various sources varies markedly in resistivity, though each within a relatively narrow range. It has also been found that the resistivity of the anthracite coal is closely related to the percentage of volatile matter therein. The following table was prepared from six samples of anthracite coal found in the State of Pennsylvania.

The electrical resistivity of the 20 mesh' anthracite coals in Table I is of an entirely different order from the resistivity which will be secured when the anthracite coal is applied in combination with a vehicle. However, the resistivity of the anthracite coal in combination with the vehicle varies almost directly with resistivity of the several powdered coals.

The compositions of this invention are com By natural anthracite coal it is intended to mean anthracite coal that has not been subjected to any decomposition processses, such as would reduce the volatile content, except mechanical operations to reduce it to proper particle size for use in semi-conducting coatings.

Anthracite coal of high quality may be purchased in No. 5 buckwheat size. In preparing semiconducting compositions, anthracite coal is prepared initially by blending two or more anthracite coals secured from diierent sources in such proportions as determined by experience that will give a mixture of predetermined resistivity. It is, of course, infrequently found that a single variety of anthracite coal is suitable for a given application of this invention.

The coal may be preground to 100 mesh iine ness if desired. The coal is subjected to milling in ball or tube mills or the equivalent in water for a period of the order of 24 hours until the coal has been reduced to a substantially colloldai condition. By milling We refer to the use of any apparatus capable of comminuting a substantial proportion of the coal to colloidal flneness. For ball milling, a 30% coal suspension in water is suggested. Tests of the physical state of the coal during the ball milling operation will indicate whether or not the coal has been reduced to suitable fineness. One suitable test is to prepare a dispersion of the ball mill product in water having. 0.01% coal by weight. The dispersion so prepared is introduced into a cell of a photoelectric colorimeter. When the light transmission of the coal dispersion is about 5%, then the ball milling may be considered satisfactory. Light transmission values of 1A of 1% to 10% are indicative of satisfactorycomminution. A 5% light transmission value is believed to correspond to an average particle size of under one micron. The product of the ball mill is strained through a fine mesh fabric in order to remove extraneous material therefrom.

The substantially colloidal coal suspended in Water is mixed with a water soluble Organic film. forming resin, gum or other substances. Gum arabic has been found particularly satisfactory for this purpose. However, gum tragacanth and gum gattie are also suitable for this purpose. Polyvinyl alcohol which has been derived by hydrolyzing the polyvinyl ester to the degree that the polyvinyl alcohol is water soluble which occurs at a hydrolysis of 70% and higher may be einployed in this application. Methyl cellulose and casein glue are other examples of suitable water soluble organic materials of this type.

As an example of the invention, the preparation of a gum arabic suspension will be described in detail. The amount of gum arabic to the colloidal coal may vary over a range of from 75 parts to 10 parts by weight of gum arabic and from 25 parts to parts by weight of coal. Sumcient water should be added to dissolve the resin and suspend the coal. customarily three or more parts of water per part of coal and resin is sufficient for this purpose. In order to stabilize the suspension and prevent deterioration of the gum, approximately 1% of ammonia based on the weight of the gum arabic is added. Other stabilizers for this purpose may be substituted. A thorough mixing of the colloidal anthracite coal, gum arabic, water, and ammonia will give a stable suspension suitable for application to members as a. semiconducting composition. A composition viscosity of 10 to 100 seconds No. 1 Demmler cup is satisfactory for application.

Referring to Fig. 1 of the drawing, there is illustrated a portion of the end windings of an electrodynamic apparatus I0 to which the semiconducting composition may be applied. The apparatus II consists of a plurality of laminations i2 of magneticv material held in position bythe end plates Il. Slots Il provided in the magnetic material l2 retain windings I8 composed of electrically insulated conductors. The conductors I3 terminate in end windings 2l in which the conductors are bent and turned for re-entry into other suitable slots. Due to the powerful electromagnetic forces present in the conductors which would distort the conductors and induce destructive vibration, it is necessary to suitably brace and tie or lash the conductors to one another and to the end winding supports. For this purpose. spacer blocks 22 which may be prepared from an insulating material such as wood or a semiconducting composition from the type disclosed in the copending patent application of Atkinson, Hill, andv Barker, Serial No. 467,728, filed December 3, 1942, are inserted between successive end windings in such manner as to properly space and support the windings from one another. 'Iies and lashings 24 are applied about successive conductors to rigidly lash the end windings to the spacer blocks 22 and thereby prevent vibration. At the point where the slot windings I3 emerge from the slot I8, channeled wedgeshaped spacers 26 are customarily. applied. 'I'hese spacers may be made similar to those satisfactory for use for blocks 22. Ties or lashings 23 are applied about the blocks 23 in the channels thereof to rigidly fix the wedge-shaped blocks in position. In some cases cross ties 33 between successive windings are applied adjacent the blocks 2l.

'I'he suspension of colloidal anthracite coal and the organic film-forming resin is preferably applied to the insulated surfaces oi' the conductors |323 prior to the assembly `oil, ythe apparatus I0. In this way, a highly uniform and complete coverage is secured. Since ordinarily the end windings 20 constitute a close and dense construction, it is quite diillcult to apply semiconducting composition after assembly of the apparatus. After the windings I 3 2. have been assembled in the apparatus and the spacers 22 and 23 applied and lashings 24, 2l, land 30 tied, the entire assembly is grounded to the slot portions of the windings I3 which are coated with a low resistance graphite coating such as aquadag which is in contact with the slot walls I3. The entire assembly may be painted with insulating weatherresisting varnish to provide for added protection from moisture and the like. 'I'he insulating weather-resistant varnish is not detrimental to the function of the semiconducting coating 32.

The coating 32 may be produced by simply applying the colloidalV coal-gum arabic water suspension as a paint to veach of the insulated conductors, one or more coatings Abeing applied as deemd desirable. Alternatively, the several insulated conductors may be wrapped with a semiconducting tape previously prepared from the semiconducting composition.

Referring to Fig. 2 of the drawing, there is illustrated in cross-section a semiconducting tape 40 comprising a fabric material 42 prepared from glass fibers, cotton fibers, or asbestos fibers, or

equivalent material as desired, the tape beingv impregnated with an aqueous suspension of anthracite coal and gum arabic. The tape may be conveniently prepared by simply dipping the fibrous material in the suspension and passing through rolls under a constant pressure to reremoved bydryingin ovens orbyexposing to infrared lamps or the equivalent. A substantially dry tape may be produced. For certain purposes, however, the tape is preferabhr dried to remove only a major proportion ofthe water, leaving a certain amount of moisture in the tape. 'Ihe moisture is maintained in the tape by storing the tape in cans having a supply of water in the bottom sufficient to maintain humidity or else a moisture-tightl cover applied to the container. Such moisture renders the tape more pliable than a completely dry tape and is advantageous for` other reasons to be detailed subsequently.

Thel tape may be applied to insulated conductors by wrapping the tape so as to overlap the successive spirals of the tape. It has been discovered that tape containing an appreciable amount-of moisture which renders the water soluble organic nlm-forming material such as gum arabic slightly tacky and plastic, causes the several convolutions of the applied tape on a conductor to attain maximum contact. When tape so applied is dried, the conductor may be employed with high voltages, and no corona will be evident at the spaces between the overlapped convolutions of tape. The entire surface of the conductor will be substantially corona-free. This advantage is not present in any tape of the semiconducting nature manufactured heretofore and constitutes a decided advantage in favor of the tape produced from the semiconducting composition disclosed herein. g

The tape 40 can be employed for the tiesor lashings 2l, 23, and 33 in addition to functioning as the semiconducting coating 32.

The semiconducting water suspension composition maybe applied to the conductors |8-20 in another manner which has certain advantages over the processes described previously. Referring to Fig. 3 of the drawing, the insulated conductor ill is initially painted with the water suspension of substantially colloidal anthracite coal and gum arabic as a coating I2. The coating 52 while still wet is wrapped with an untreated porous fibrous tape Il of glass fibers, cotton fibers or asbestos ilbe'rs or the like. The tape il is applied with suillcient pressure to 'cause the lsemiconducting coating 52 to penetrate into the tape and ooze through the intersticesthereof. Thereafter an outer coating 53 of the colloidal suspension is applied over the tape 54. The resultant semiconducting coating is exceedingly durable and resistant to normal types. of abuse and wear and tear. 'I'he tape 54 prevents the semiconducting compound from aking oil or peeling and is exceedingly advantageous.

The blocks 22 and 2l, if of wood or other electrical insulating material, may be rendered semiconducting at the surfaces thereof in order to reduce the potential between successive end windings by applying a coating of the colloidal suspension thereto. On drying, the blocks may be inserted between windings in the customary manner. Y

As previously mentioned, the entire assembly I0 may be dipped, sprayed, or brushed with a heavy coating of weather-resistant' paint or enamel. The paint or enamel is advantageous in that it reduces the eiTect of changes in atmospheric humidity on the water soluble resins.

As an example of the remarkable uniformity and stability of the semiconducting compounds, a coil was prepared by brushing the colloidal .suspension as a 3 to 4 mils thick coating over the move excess suspension. The moisture may be 73 insulation and a heavy insulating and weather conducting coatings of this invention are exceedingly stable with time even when subjected to elevated temperatures.

When tape is applied to various electrical members, an adhesive may be applied to the surface of the conductor in order to eliminate any possibility of a gas retaining void below the several convolutions of tape. A solution of polyisobutylene or high grade asphalt or other coating substance may be applied to the surface of the coils. and as the`tape is applied, the polyisobutylene, for example, will fill the voids under the tape. A further advantage of this modified form oi the invention is that of better adhesion oi the tape to the conductor.

Semconducting tape prepared according to this invention was applied to coils without separately painting the coils before or after the application of the tape, the tape being applied in a humid or moist condition. The dry resistivity ot the tape was 20 megohms, the tape being made of 10 mils thick glass cloth with the suspension applied to a total thickness of 12 mils. 4The coils to which the tape was applied were rated at 13,800 volts. A test voltage of 29,000 volts was applied for one minute without any sign of corona. Subsequently 40,000 volts were applied for a considerable period of time without the observance of any corona at any point on the coils. This high voltage had no ill efi'ect on the tape surfaces of the coils.

The advantages of the gum arabic and the other substantially nonreactive resins set forth in producing semiconducting coatings over polymerizing resins resides in the attainment of a substantially constant resistance immediately upon drying. Polymerizlng resins may take months at operating temperatures to reach a reasonably steady state. The resistivity of such polymerizing resin coatings may easily change twenty-fold from a fully dried condition over a period oi30 days at 100 C. The gum arabic type of composition has a long life 'with a nearly constant resistivity.

In the application of semiconducting coatings to end windings and other electrical apparatus, it is highly desirable to secure a. reasonable degree of uniformity of resistivity between various portions of the surfaces thereof. Commercial quality control 4has been attained by the practice of the invention described herein easily, conveniently and economically. By employing colloidal anthracite coal-gum arabic suspensions, for example, a maximum variation in resistivity be tween any portion of an end winding of the order of :20% has been found commercial practical.

Since certain obvious changes may be made in the above procedures and diierent embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or taken in connection with the accompanying drawing. shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A composition suitable for applying to mem- 1| ing take place.

' conducting coating applied to the insulation, the

' coating composed of natural anthracite coal havbers to produce a semiconducting coating thereon comprising. in combination, a colloidal suspension oi natural anthracite coal having less than 10% volatile matter, the suspension derived by combining substantially colloidal natural an thracite coal, water, a Water soluble organic film forming agent and a stabilizer.

2. A composition suitable for applying to members to produce a semiconducting coating thereon comprising, .in combination, from 25 to 90 parts by weight of a colloidal suspension of natural anthracite coal having less than 10% voletlle matter, the suspension being derived by milling natural anthracite coal in Water, from to l0 parts by Weight of a water soluble organic film-forming agent, water and a stabilizer.

3. A composition suitable for applying to members to produce a semiconducting coating thereon comprising, in combination. from 25 to 90 parts by weight of a colloidal suspension ci natural anthracite coal having less than 10% volatile matter, the suspension being derived by milling natural anthracite coal in water, from 'l5 to 10 parts of gum arabic, water and a stabilizer.

4. A semiconducting flexible tie member, comprising an elongated fibrous body and a semiconducting coating applied thereto, the coating composed of ilnely divided natural anthracite coal l having less than 10% volatile derived by milling the coal in water to a substantially colloidal state and a water soluble organic ilimforming binder for the anthracite coal.

5. A semiconducting iiexibie tie member, comprising an elongated nbrous body and a semiconducting coating applied thereto, the coating composed of from 25 to 90 parts by weight of ilnely divided natural anthracite coal having less than 10% volatile andderived by milling the natural anthracite coal in water to a substantially colloidal state and from 10 to 75 parts by weight of a water soluble organic film-forming binder for the anthracite coal.

6. A serniconducting ilexible tie member, comprising an elongated fibrous body and a semiconducting coating applied thereto, the coating composed of from 25 to 90 parts by weight oi' finely divided natural anthracite coal having less than 10% volatile and derived by milling the natural anthracite coal in water to a substantially colloidal state and from 10 to '75 parts by weight of a water soluble organic nlm-forming binder composed o! gum arabic for the anthracite coal.

7. A semlconducting flexible tie member, comprising an elongated nbrous body and a semiconducting coating applied thereto, the coating composed of nely divided natural anthracite coal having less than 10% volatile derived by milling the natural anthracite coal in water to a substantially colloidal state, a Water soluble organic nlm-forming binder for the anthracite coal, and water in an amount sufficient to provide for plasticity in the binder.

8. An insulated conductor suitable for use at high voltages without corona being formed at the surface of the insulation exposed to air or other gases, comprising, a conductor, electrical insulation applied to the conductor, and a semilng less than 10% volatile matter of substantially colloidal. ilneness distributed in a water soluble binder, the binder being substantially nonreactive when applied as a film whereby no substantial changes in characteristics of the coat- 9. An insulated conductor suitable for use at high voltages without corona being formed at the surface of the insulation exposed to air or other gases, comprising, a-conductor, electrical insulation applied to the conductor, and a semiconducting coating applied to the insulation, the coating composed o! from 25 parts to 90 parts by weight of natural anthracite coal having less than 10% volatile matter of substantially colloidal neness distributed in from 75 parts to 10 parts by weight of a water soluble binder, the binder being substantially nonreactive when ap- 

